This invention relates generally to compositions and methods for forming etch resistant masks, and more particularly to radiation sensitive negative resists for semiconductor device fabrication.
A resist is an adhering layer of a material on a support in which a pattern of openings are formed, through which the support is etched. The use of light as the radiation source to form the pattern in the resist is well known and was generally adequate until the advent of small geometry integrated circuits which require line widths on the order of 1 micron. Although 1 micron resolution can be obtained using photolithography in the laboratory, such line widths are generally not reproducible because of diffraction problems, the practical limit in a production environment being about 3 microns.
More recently, e-beam and x-ray lithography have been used to fabricate very large scale integrated circuit chips. In the case of e-beam technology, the beam is scanned across the resist to form the desired pattern. The e-beam may be controlled by a computer which has been programmed with the coordinates of a predetermined pattern. The use of the e-beam has thus eliminated the time lost in preparing a photographic reduction required to form the pattern. A disadvantage of this process is the long scan time necessary to expose the pattern in the resist, especially for very large scale integrated circuits. A resist which permits very high scanning speeds is necessary in order to use e-beam technology in a production environment. X-ray technology involves a flood exposure of the resist through a mask similar to that used in optical lithography, however, much narrower line widths can be formed.
Conventionally, there are two kinds of resists, positive and negative working resists. A resist whose exposed portion is more soluble in a developer than its unexposed portion is a positive resist. With a positive resist the exposed area is removed after exposure and developing. A resist whose exposed area is less soluble than the unexposed area is a negative resist, in which case the exposed area is retained after exposure and developing.
Resists suitable for one lithographic application are generally unsuitable for others. For example, a photoresist containing cyclolized polyisoprene and bisazide is very sensitive to ultraviolet radiation in the range of 300-450 m.mu.. Conversely, epoxidized materials which are known to be very sensitive to e-beam radiation are relatively insensitive to ultraviolet radiation.
Liquid developers are commonly used to remove the more soluble portion of resists after exposure. These developers may be aqueous basic solutions for positive resists, or organic solvents for both positive and negative resists. One problem encountered in using liquid developers is that they tend to penetrate the less soluble portion of the resist and cause swelling of the resist pattern. The extent of the swelling is minimal for most positive resists, but it is substantial for the crosslinkable negative resists. A low degree of solvent penetration may cause the resist to have a rough edge after drying, and for higher penetration the adjacent geometries may run together and collapse upon development, thus partially filling gaps in the pattern that are supposed to be open. Developer penetration and swelling are the major limitations to the resolution obtainable with most negative resists currently n use. Still another problem is the cost and disposal of the chemicals used in liquid developers.